1. Field of the Invention
The present invention broadly relates to methods of making ceramic composite bodies having one or more shaped cavities therein. In particular, the invention relates to methods of making ceramic composite bodies comprising a polycrystalline ceramic matrix infiltrating a bed of filler and having at least one cavity of selected geometry formed by inversely replicating the shape of an expendable pattern.
2. Background and Description of Commonly Owned Patent Applications
The subject matter of this application is related to that of copending and commonly owned U.S. patent application Ser. No. 823,542, filed Jan. 27, 1986 entitled "Inverse Shape Replication Method of Making Ceramic Composite Articles and Articles Obtained Thereby" in the name of Marc S. Newkirk et al. This copending application discloses a novel oxidation method for producing a self-supporting ceramic composite body having therein at least one cavity which inversely replicates the geometry or shape of the parent metal precursor as the positive pattern. thus, the resulting composite product has a cavity of a predetermined geometry. This application is discussed in greater detail below. Composites generally utilizing the same oxidation phenomeon, but having no defined or predetermined configuration, are disclosed in copending and Commonly owned patent application Ser. No. 819,397, filed Jan. 17, 1986, which is a continuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, both in the name of Marc S. Newkirk et al and entitled "Composite Ceramic Articles and Methods of Making Same." These copending applications disclose a novel method for producing a self-supporting ceramic composite by growing an oxidation reaction product from a parent metal into a permeable mass of filler.
The method of growing a ceramic product by this oxidation reaction is disclosed generally in copending commonly owned U.S. patent application Ser. No. 818,943 filed Jan. 15, 1986 as a continuation-in-part of Ser. No. 776,964, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 705,787, filed Feb. 26, 1985, which is a continuation-in-part of Ser. No. 591,392, filed Mar. 16, 1984, all in the name of Marc S. Newkirk et al and entitled "Novel Ceramic Materials and Methods of Making the Same." These applications disclose the method of producing self-supporting ceramic bodies grown as the oxidation reaction product of a parent metal precursor which may be enhanced by the use of an alloyed dopant. Molten parent metal is reacted with a vapor-phase oxidant to form the oxidation reaction product. In the appropriate temperature range, molten metal is progressively drawn through the oxidation reaction product and into contact with the oxidant thereby continuing to form additional oxidation reaction product and developing the ceramic body. The method was improved upon by the use of external dopants applied to the surface of the precursor parent metal as disclosed in commonly owned U.S. application Ser. No. 822,999, filed Jan. 27, 1986, which is a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 747,788, filed June 25, 1985, which is a continuation-in-part of Ser. No. 632,636, filed July 20, 1984, all in the name of Marc S. Newkirk et al and entitled "Methods of Making Self-Supporting Ceramic Materials".
The entire disclosures of each of the foregoing commonly owned patent applications, which are assigned to the same owner, are expressly incorporated hereby by reference.
There is an increased interest in substituting ceramics for metals because, with respect to certain properties, ceramics are superior to metals. There are, however, several known limitations or difficulties in making this substitution such as scaling versatility, capability to produce complex shapes, satisfying the properties required for the end use application, and costs. The above-described commonly owned patent applications overcome these limitations or difficulties and provide novel methods for reliably producing ceramic materials, including composites.
The invention described in commonly owned patent application Ser. No. 823,542 (identified above) ameliorates the difficulties in formation of ceramic bodies having shapes with complicated internal cavities and especially shapes with re-entrant cavities. Conventional or known methods for making ceramic products with such shapes by compacting and inserting particles are not readily applicable, because the internal pattern required to establish the desired part geometry cannot be easily removed after the body is formed around it. While such part geometries can sometimes be prepared by machining or grinding the desired shape from green preform or finished ceramic blank, this approach is undesirable because of the high costs of machining and grinding operations, especially when applied to ceramic materials. In many cases such geometries cannot presently be produced at all, even by machining or grinding.
The invention described in commonly owned patent application Ser. No. 823,542 provides shaped cavity-containing ceramic bodies typically of high strength and fracture toughness by a mechanism which is more direct, more versatile and less expensive than conventional approaches. The invention described therein also provides means for reliably producing cavity-containing ceramic bodies of a size and thickness which is difficult or impossible to duplicate with the presently available technology. Briefly, the invention therein described involves embedding a shaped parent metal precursor within a conformable filler, and infiltrating the filler with a ceramic matrix obtained by oxidation of the parent metal to form a polycrystalline material consisting essentially of the oxidation reaction product of said parent metal with an oxidant and optionally, one or more metallic constituents. More particularly, in practicing the invention, the parent metal is shaped to provide a pattern, and then is surrounded by a conformable filler wihch inversely replicates the geometry of the shaped parent metal. In this method, the filler (1) is permeable to the oxidant when required as in the case where the oxidant is a vapor-phase oxidant and, in any case, is permeable to infiltration by the developing oxidation reaction product; (2) has sufficient conformability over the heat-up temperature interval to accommodate the differential thermal expansion between the filler and the parent metal plus any melting-point volume change of the metal; and (3) at least in a support zone thereof enveloping the pattern, is intrinsically self-bonding, whereby said filler has sufficient cohesive strength to retain the inversely replicated geometry within the bed upon migration of the parent metal as described below. The surrounded shaped parent metal is heated to a temperature region above its melting point but below the melting point of the oxidation reaction product to form a body of molten parent metal. The molten parent metal is reacted in that temperature region or interval with the oxidant to form the oxidation reaction product. At least a portion of the oxidation reaction product is maintained in that temperature region and in contact with and between the body of molten and the oxidant, whereby molten metal is progressively drawn from the body of molten metal through the oxidation reaction product, concurrently forming the cavity as oxidation reaction product continues to form within the bed of filler at the interface between the oxidant and previously formed oxidation reaction product. This reaction is continued in that temperature region for a time sufficient to at least partially embed the filler within the oxidation reaction product by growth of the latter to form the composite body having the aforesaid cavity therein. Finally, the resulting self-supporting composite body is separated from excess filler, if any.